The goal of this research is to evaluate the specificity of some purified esterases as markers of monocytes. The availability of such markers would allow for a much needed analysis of histiocytic neoplastic disease. The esterases in monocytes were separated into three major enzymes of DEAE-cellulose column chromatography followed by CM-sepharose column chromatography. DEAE separated the esterases into peaks I and II. CM-sepharose separates peaks I into peaks IA and IB. Peak IA is highly specific to acetyl ester of aromatic alcohol, while peak IB and peak II are nonspecific to the molecular structure of the acyl group. Antisera specific to IB and II have been produced. Immunodiffusion data indicate that the three esterases are totally different protein species.